Pressurized flotation module and method for pressurized foam separation

ABSTRACT

A method and several embodiments of apparatus are disclosed for use in foam flotation separation. The method discloses performing the separation in a module operated at nonatmospheric pressure, and the apparatus disclosed are suitable for use as the module of the method. Both the method and the apparatus contemplate positive and negative pressure operation.

BACKGROUND OF THE INVENTION

i. Field of the Invention

The present invention relates generally to froth flotation separationand has particular use in the field of deinking of paper in a processfor recycling waste paper. More specifically, the invention concerns anapparatus and method for separating ink-laden foam from a slurry of thepaper stock.

ii. Description of the Related Art

Froth flotation is a subdivision of a general separation technique knownas adsorptive bubble separation. In froth flotation, particles areselectively separated by the differing abilities of different particlesto adhere to air bubbles rising through a bulk liquid, and to be held atthe liquid surface in a foam or froth.

Flotation separation is frequently used in a variety of processes forvarious purposes. For example, selective flotation can be used toseparate minerals, and total flotation is used for clarification byremoving all solids. Recently, a developing application for flotationseparation has been for ink removal in the field of paper recycling.

Various factors have contributed to development in the art of recyclingwaste papers for the purpose of repulping the paper to form paper.Recycling makes use of a readily available source of materials, thewaste paper or discarded papers which would otherwise present disposalproblems. Normally, this represents a very inexpensive and readilyavailable source of material, thus providing an economic incentive forthe papermaker to recycle waste paper. Increased ecological awarenesshas resulted in a demand by various populations to recycle paper,thereby eliminating the disposal problem and the resultant ecologicalimpact. Therefore, there are incentives for the papermaker to encourageconsumers to recycle paper and for the papermaker himself to userecycled fiber. Use of recycled fiber often can provide a less expensivepaper product than using virgin fiber, and there is an economicincentive on the part of consumers to encourage papermakers to userecycled fiber as well as for the consumer himself to make availablewaste paper for recycling.

The process of waste paper recycling is essentially one of removingcontaminants from usable fiber. In the deinking of wastepaper, which isa significant portion of the overall wastepaper recycling market, one ofthe critical steps is removing ink from a slurry of pulp fibers.Typically, the paper to be recycled is repulped in some fashion, and aslurry of the recycled fibers is made. Chemicals are added to separatethe ink particles from the fiber, and to disperse the ink Particlesfreely in the pulp slurry. Various different processes and chemicals canbe used, which do not constitute a part of the invention and will not bedescribed in further detail.

After the ink has been separated from the fiber it must be removed fromthe pulp slurry. Two processes for removing the ink particles from theslurry have been used by most recyclers, the selection of which oftendepends on various characteristics of the fiber being recycled and theink to be removed. A first process for removing the ink particles isoften referred to as dilution washing which essentially rinses the inkparticles from the pulp.

A second type of ink separation is generally referred to as frothflotation. In froth flotation separation, pulp of low consistency ispassed through a series of flotation cells. Air is mixed with the stockalong with flotation chemicals such as fatty acids or surfactants. Theink particles have a greater affinity for the air bubbles than do thepaper fibers, and the ink particles attach to the bubbles. The bubblesrise to the surface in the flotation cell, forming an ink-laden frothwhich can be continuously removed from the slurry surface. Removal ofthe froth is accomplished by gravity, by vacuum or with paddles or otherskimming devices at the surface. The froth flotation process can also beused for removing various adhesive contaminants known as stickies fromthe pulp slurry as well.

Generally, less than thirty per cent of the total ink present in theslurry will be removed in a single froth flotation cell. Therefore, ithas been a common practice in the past to provide four to six flotationcells in a series to remove an acceptable portion of the ink originallypresent in the repulped slurry. Each deinking cell requires variousvalves, pumps and associated apparatus including piping for providingslurry to the cell, the air injecting and mixing apparatus, chemicaladdition, froth removal and slurry removal. Therefore, the capitalinvestment necessary for a system to remove the required portion of theink from recycled fiber can be quite high.

Known froth flotation cells are all of an atmospheric type. That is, theslurry in the cell is at atmospheric pressure. The air injected into theslurry to float the ink particles is allowed to escape to theatmosphere. A typical example of the heretofore known froth flotationdeinking cells can be found in U.S. Pat. No. 4,548,673. While a cover isoften provided on the deinking cell, the cell remains to operate atatmospheric pressure. For example, U.S. Pat. No. 4,328,095 discloses aseries of covered chambers; however, the chamber operates at atmosohericpressure.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide apressurized flotation module for removing froth from a slurry in anefficient manner, and which can eliminate much of the piping, pumps andvalves heretofore necessary in froth flotation processes.

Another object of the present invention is to provide a pressurizedflotation module for froth flotation deinking processes which can beoperated in a single unit to achieve suitable contaminant removalwithout requiring duplication of froth removal apparatus.

Still another object of the present invention is to provide apressurized flotation module for deinking of pulp slurries which can beoperated at pressure above atmospheric pressure, and in which the airescaped from the slurry can be caused to perform useful work.

A still further object of the present invention is to provide apressurized flotation module for deinking pulp slurry which reduces thefoam reject volume and reduces yield losses when compared withheretofore known types of froth flotation cells, and which is readilyadaptable for design to various system size requirements, including highvolume systems, with minimal space requirements.

These and other objects are achieved in the present invention byproviding a flotation separation method and a flotation module which canbe operated under nonatmospheric pressure in place of the heretoforeknown atmospheric flotation methods and modules. The flotation module ofthe present invention can encompass many different shapes and includesan inlet receiving stock from a mixing zone in which air and/or foamingagents are added to the slurry. An outlet is provided for the acceptableportion of the stock, and a foam outlet is provided for removing theink-laden foam from the slurry. In preferred embodiments of the module abaffle is provided between the foam and stock outlets, to direct stocktoward the stock outlet and the ink-laden foam toward the foam outlet.Valves control the flow rate at the stock outlet and the foam outlet. Tosimulate the operation of consecutive deinking cells, various portionsof the accepted slurry can be recirculated to the inlet end. The methodand module can be operated under positive or negative pressure.

Additional objects and advantages of the invention will be apparent fromthe following detailed description and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a deinking system utilizing a pressurizedflotation module according to the present invention;

FIG. 2 is a longitudinal cross sectional diagramatic view of apressurized flotation module according to the present invention, showingslurry in the module;

FIG. 3 is an enlarged cross sectional view of a pressurized flotationmodule, taken generally along line 3--3 of FIG. 2;

FIG. 4 is an enlarged longitudinal cross sectional view of the outletcone of the pressurized flotation module;

FIG. 5 is an enlarged longitudinal cross sectional view of the inletcone of the pressurized flotation module;

FIG. 6 is an enlarged longitudinal cross sectional view of the centralbody portion of the pressurized flotation module;

FIG. 7 is a vertical cross sectional view of an alternative embodimentfor a pressurized flotation module;

FIG. 8 is a top view of the module shown in FIG. 7;

FIG. 9 is a vertical cross sectional view of yet another embodiment fora pressurized flotation module of the present invention;

FIG. 10 is a top view of the module shown in FIG. 9; and

FIG. 11 is a vertical cross sectional view of still another embodimentfor a pressurized flotation module.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now more specifically to the drawings and to FIG. 1 inparticular, numeral 10 designates a froth flotation system in which apressurized flotation module 12, according to the present invention, canbe utilized. System is shown to be a system for deinking recycled wastepaper; however, the module 12 can be used for other purposes such asmineral separation, fluid clarification and the like. In the deinkingsystem 10, waste paper stock 13 from a pulper or the like is supplied toan inlet stock tank 14, which includes agitators or the like to maintaina consistent suspension of the fibers in the slurry. A supply line 16fed by a pump 17 extends between the inlet stock tank 14, and an inletend 18 of the pressurized flotation module. Those skilled in the artwill recognize that the inlet of the module can be fed other than by apump. For example, gravity feed can also be used. At the opposite end ofthe pressurized flotation module from the inlet end 18 there is disposedan outlet end 19, including a deinked pulp outlet 20 and an inky foamrejects outlet 22. A recycle loop 24 extends between a deinked pulpoutlet line 26 and the inlet stock tank 14. The recycle loop 24 permitsa controllable portion of the deinked stock to be recycled, therebymaintaining the desired level of ink removal without having to provideseries of deinking apparatus. A rejects line 27 is provided for carryingthe ink-laden foam away from the flotation module. The operation of thesystem utilizing a pressurized flotation module 12, according to thepresent invention, and particularly the recyclinq of a portion of thedeinked slurry will be described in more detail hereinafter.

Various system controls such as valves 28, 30 and 32 are disposed in thefoam rejects line 27, the deinked pulp line 26 and the recycle loop 24,respectively. Various other flow control valves, pumps and associatedapparatus may also be required, all of which would be known by oneskilled in the deinking art and will not be described in further detailherein.

Referring now more specifically to FIG. 2, and particularly the enlargedviews in FIGS. 3, 4, 5 and 6, one embodiment of the pressurizedflotation module 12 will be described in more detail. In FIGS. 2 through6, the flotation module is shown to include a generally cylindrical mainbody portion 40, an inlet cone 42, and an outlet cone 44. It should beunderstood, however, that the flotation module, according to the presentinvention, can be constructed in different shapes such as elliotical orconical as well as the generally cylindrical shape shown, and thepresent description and claims are intended to include other such shapesas well. Further, while a generally horizontal arrangement is shown forthe module, it should be understood that with minor modification a moregenerally vertical arrangement can be used. Several alternativeembodiments of the pressurized flotation module will be described laterherein.

The inlet cone 42 of module 12 is connected to an aeration mixingsection 50, which receives the slurry from the inlet stock tank, andincludes an air injection device 52, and a turbulent mixing section 54.A suitable type of aeration and mixing section for use with the presentpressurized flotation module is shown in co-pending application U.S.Ser. No. 898,475 (now U.S. Pat. No. 4,861,165). It should be understoodthat various other forms of aeration and mixing sections can also beused.

Inlet cone 42 includes a first flange 60 with a plurality of bolt holes62 or other suitable means for connecting the inlet cone to the supplyline 16. A second flange 64 containing a plurality of bolt holes 66 isdisposed at the opposite end of the cone for connecting the cone to body40 of the pressurized flotation module. Extending between flange 60 andflange 64 is a generally diverging wall 68 of the inlet cone. The angleof expansion of wall 68 is important in that, if the angle is too great,the slurry flowing into the pressurized flotation module from the supplyline may generate undesirable eddies and flow patterns, disruptingoperation of the flotation module. It is desirable that the major flowpath through the module be linear, so that the ink-laden bubbles in theslurry can rise to the top of the slurry flowing through the flotationmodule without being caught up in, or ruptured by eddies in the slurry.For these reasons, it has been found that an angle of expansion for theinlet cone of 40° or less is desirable, and preferably the angle ofexpansion will be less than 30°. The gradually expanding wall 68 slowsthe fluid velocity of the slurry as it enters the module, therebyallowing the bubbles in the slurry to rise through the slurry.Alternatively, diffusers can be used to slow the fluid velocity.

Body 40 includes flanges 70 and 72, each having a plurality of boltholes 74, for connecting the body to the inlet and outlet conesrespectively. As mentioned previously, body 40 can be of many differentshapes, generally defined by a wall 76 formed in, for example, thecylindrical shape shown, an elliptical shape or a generally divergingcone shape. The body also includes legs 78 and 79 for supporting themodule. Generally, it has been found that a gradual upward rise from theinlet end to the outlet end is desirable, at least along the upperportion of body 40. Therefore, leg 79 is generally longer than leg 78.In one model of the invention, a 6" rise over a 7' body 40 length wasfound to be appropriate.

Outlet cone 44 includes a flange 80 with a plurality of bolt holes 82for connecting the outlet cone to the body 40. It will be understood bythose skilled in the art that other than bolt-type connections may beused for securing the inlet and outlet cones to the body 40, and forsecuring the cones to the various slurry conduits in the system. Withthe flange and bolt-type connections, normally a gasket, not shown, willbe disposed between the adjacent flanges to provide a positive seal.

Outlet cone 44 further includes a generally horizontally extendingoutlet wall 90, having a flange 92 with a plurality of bolt holes 94,for connecting the outlet cone to the deinked pulp line 26. Extendingbetween the inner end of wall 90 and flange 80 is a conical wall 96.Disposed in wall 96 on the upper portion thereof, near the outlet wall90, is a flange housing 98 for connecting the outlet cone to theink-laden reject line 27.

To separate the slurry accepts outlet from the reject outlet a baffleassembly 100 is disposed in the pressurized flotation module, andincludes an angularly upwardly extending baffle plate 102 projectinginwardly from wall 90, having a weir plate 104 on its distal end. Aswith the angular wall of the inlet cone 42, the baffle plate 102 shouldform an angle from the horizontal of approximately 40° or less, andpreferably less than 30°. If the angle from the horizontal of the baffleplate 102 is too great, undesirable flow patterns are generated near theoutlet end, which can cause air and/or foam rejects to be drawn alongwith the acceptable slurry through the accepts outlet. It has been foundthat an angle of less than about 30° from the horizontal substantiallyminimizes turbulence and the formation of eddies, and promotes thedesirable linear flow through the outlet.

Weir plate 104 extends substantially vertically upward from the baffleplate and provides a zone along its length, which accommodates anddampens surface waves in the module and allows for fluctuation in theslurry level. With the upper portion of wall 96, the baffle assembly 100forms a substantially enclosed rejects outlet chamber 110, with the onlyopening thereto being a throat 112 above weir plate 104.

In the method of the present invention, a gas is mixed with a slurrycontaining at least one constituent to be removed. The slurry with thegas dispersed and dissolved therein is fed to an enclosed vessel whereinthe gas is allowed to rise to the surface of the slurry, carrying theconstituent with it, and generating a foam on the slurry. Gas escapingfrom the slurry and foam is captured in the vessel, and forces the foamthrough a rejects outlet, with the slurry flowing through an acceptsoutlet. Flow control is maintained on at least one of the slurry inletor the rejects outlet to control the flow and pressure in the module. Asa variation to the method, the invention contemplates providing vacuumat the rejects outlet to maintain a negative pressure in the module.

In the use and operation of a pressurized flotation module according tothe present invention, particularly one such as module 12 shown in FIGS.2 through 6, and in a deinking system as disclosed in FIG. 1, slurryfrom stock tank 14 is pumped to the flotation module through theaeration and mixing section 50. Air is injected into the slurry and isturbulently mixed therewith in the turbulent mixing section 54. Theaerated slurry enters the flotation module through inlet cone 42. Thegentle angle of expansion in the inlet cone permits a gradual dispersionof the slurry into the module, with a corresponding decrease in fluidvelocity. As the slurry flows under pressure through the module, theink-laden bubbles rise to the surface of the slurry. The various valvesand pumps on the inlet and outlet lines are controlled to maintain afluid level in the flotation module such that the fluid is above thebottom of weir plate 104 and below the top of the weir plate. As thezone above the surface of the slurry becomes filled with the foam, thefoam is forced through throat 112 into the outlet chamber 110 by the airpressure created as air escapes from the slurry. The pressure forces thefoam through the reject outlet without the need for pumps, paddles,vacuum or the like. It is desirable to keep the slurry level in thegeneral area of the height of weir plate 104. If the slurry level risesabove the top of the weir plate, acceptable fiber will be lost out thefoam reject outlet. Preferably, the slurry level is sufficiently belowthe weir plate that surface waves will not crest over the top of theweir plate. If the slurry level falls below the bottom of weir plate104, ink-laden foam can be drawn along with the acceptable slurrythrough the outlet 20.

A pressurized flotation module such as that just described can beoperated with stock consistencies from about 0.1% to 5% solids, and canbe used on all raw materials with any deinking chemistry. Thepressurized module is able to efficiently handle high foaming stocks.One particular advantage of the present flotation module is that thefoam laden reject flow is under pressure and is forced through a controlvalve 28. The high shear experienced by the foam in the control valvebreaks down the foam at the valve so that the outlet from the valve is aliquid which is much easier to handle than the foam obtained inatmospheric flotation modules.

As mentioned previously, another of the distinct advantages of thepresent invention is that, through the use of recycle loops, apressurized flotation module of minimal physical size can be used toremove the desired amount of contaminant. This is a result of thegeometry presented by a the pressurized module, wherein the velocitythrough the module equals the flow rate ÷π×r². Thus, an increase incapacity is achieved by an increase in the radius, and since capacityvaries with the square of the radius, a small increase in the radius ofthe module results in a substantial capacity increase.

By way of example, a 100 tons per day module may be slightly greaterthan three feet in diameter. In scaling this module up to the 500 tonsper day capacity, the diameter would increase to only between four tofive feet in diameter. As is known with other types of froth flotationcells, as the depth of the slurry increases, an increased length of flowin the module is required to provide sufficient slurry retention time topermit the bubbles to rise from the bottom of the slurry to the top ofthe slurry. Therefore, in addition to the moderate increase in theradius of the module of the present invention, as the module is scaledup for larger capacity, the length of the body 40 would also beincreased to increase the slurry retention time.

As a result of the ease in which the module can be scaled upward forlarger capacities, instead of using, for example, five 100 tons per daymodules in series, as is frequently done with atmospheric flotationcells, a 500 ton per day module can be used with an 80% recycling of theflow from the accepts outlet 20. Alternatively, the accepts outlet of asmall unit can be hooked directly to a second turbulent mixing sectionleading to a second flotation module. Even when a series of pressurizedmodules are used, capital savings are realized in that, since pressureis maintained in the module, pumping is not required between stages, aswith atmospheric flotation cells in which pressure is lost in the cell.Normally, however, it is believed to be more efficient to recycle stockand use a larger module. A pressurized flotation module, according tothe present invention, can be used in a deinking process in place ofeleven stages of conventional atmospheric cells, so that, for example, amodule of 1100 ton per day capacity, to achieve better than 90% cleaningefficiency, would recirculate 1,000 tons per day and produce 100 tonsper day of cleaned slurry.

As mentioned previously, there are numerous variations possible in thephysical shape of the pressurized flotation module. In FIGS. 7 and 8 amodified embodiment is shown in which the module 100 is generally morevertically oriented than the module shown in FIGS. 2 thru 6. Module 100includes an outer housing or wall 102, having a slurry inlet 104 and aslurry outlet 106 disposed in the lower portion thereof. Generally, itis preferred that the slurry inlet be located somewhat higher than theslurry outlet to prevent short-circuiting of contaminants directly fromthe inlet to the outlet. A foam reject pipe 108 is disposed in the upperportion of module 100 and includes an elongated orifice 110 along thebottom portion of the pipe, which orifice extends from the end of thepipe within the module to near the inner surface of wall 102. Suitablecontrol mechanisms such as a valve 112 on the reject pipe are provided.In this embodiment, the contaminant-laden bubbles rise to the surface ofthe slurry, which is kept below the level of the reject pipe 108. Thepressure in the module forces the foam from the surface into the rejectpipe and out of the module for disposal.

A further embOdiment 120 of the pressurized module is shown in FIGS. 9and 10. This embodiment is similar to module 100, having a outer wall102, a stock inlet 104 and a stock outlet 106. The rejects are handledsomewhat differently in module 120 in that a baffle assembly 122 isdisposed in the upper portion of the flotation module, and includes anangularly upwardly extending plate section 124, projecting inwardly fromthe wall 102 and a substantially vertical weir plate 126 extendingupwardly from the inner end of the plate 124. The baffle assembly withthe module housing forms a reject outlet chamber generally designatedwith numeral 128. Foam enters the generally conically shaped chamber 128by flowing over the top of weir plate 126. The pressure in the moduleforces the foam from the module through outlet 130.

Yet another modification for a vertical module is shown in FIG. 11. Thepressurized flotation module 150 of this embodiment includes a lowerconical housing 152, tapering towards a stock outlet 154. Rising abovethe conical housing is a substantially cylindrical housing 156, whichincludes the stock inlet 158. A dome 160, having a sight glass 162, isprovided on top of the module. A central rejects pipe 164 is disposed inthe module from near the top of the module, and extends out the bottomof the lower conical housing. In this embodiment a particularlyadvantageous baffle assembly 166 is shown, which includes a flexiblebaffle membrane 168 and a floating baffle ring 170. The ring 170 floatson the surface of the slurry, with a portion thereof rising above theslurry to prevent slurry from flowing into the rejects pipe. The foamgenerated on the surface of the slurry flows over the top of the bafflering and out the reject pipe. The flexible membrane 168 can fold tooverlap itself, and the baffle assembly shown herein automaticallycompensates for fluctuations in slurry surface level. This embodimentworks similar to the previously described embodiments wherein the slurryenters the module, the bubbles rise to the surface of the slurry andflow over the baffle assembly into the rejects pipe. Acceptable slurryflows to the bottom of the module and out the outlet 154. The module isenclosed and the escaping air from the slurry is captured in the moduleand utilized to force the foam through the reject pipe.

The method and pressurized modules disclosed herein have application inseparation processes other than deinking. For example, flotationseparation techniques utilizing the present invention can be used toseparate other contaminants, such as plastic, from pulp slurries, andcan be used in mineral separation or the like. Clarification can beachieved by total flotation, which is substantially simplified byrecirculation made possible in the present pressurized flotation module.

Further, while the invention has been described for operation atpositive pressure, it is also possible to operate the flotation moduleat negative pressure. In this modification the rejects line would beconnected to a vacuum source, and the area in the module above theliquid level would be maintained at a negative pressure. In this regardthe invention varies from previous flotation apparatus wherein vacuum isused to skim foam off the slurry surface. In the present invention theentire module is enclosed. The present flotation module exhibitsversatility, being operable over a pressure range of about minus 10 topositive 100 psi absolute pressure.

While various embodiments of a pressurized flotation module, and of amethod for separating foam from a slurry under pressure have beendescribed in detail herein, various changes may be made without itdeparting from the scope of the present invention.

We claim:
 1. A flotation module for separating ink-laden bubbles from anaerated pulp slurry flowing through said module, said modulecomprising:a body defining a vertically oriented cone forming aslurry-conducting zone in which air bubbles from said aerated slurry arepermitted to rise through the slurry as the slurry flows through saidzone; an inlet means connected to said body and connected in flowcommunication with a slurry supply source; a slurry outlet meansconnected to said body for directing slurry flowing from said zone; abubble outlet means connected to said body for directing said bubbles ina flow from said zone; said inlet means and said outlets means and saidbody defining a substantially closed pressure vessel; flow control meansdisposed in flow communication with at least one of the inlet means, theslurry outlet means and the bubble outlet means to control the flow ofslurry and bubbles to generate a slurry surface in said zone foraccumulating bubbles thereon, said flow control means also controllingthe flow of air from said module for maintaining the internal pressurein the module above atmospheric pressure; said body defining said coneconverging from said inlet means to said slurry outlet means, saidslurry outlet means being downwardly disposed; said bubble outlet meansincluding a pipe extending within said cone; and said bubble outletmeans further including a floatable baffle assembly connected to saidpipe for limiting the flow of slurry into said pipe.
 2. A foamfloatation module for separating by flotation from a slurry fedtherethrough, foam generated by mixing a gas with the slurry, saidmodule comprising:a sealed, horizontal, elongated body having a slurryinlet means near one end of said body, a slurry outlet means near theother end of said body, and a foam outlet means, each spaced apart fromeach other;said slurry inlet means including a generally conicallyshaped wall diverging outwardly in the direction of slurry flow; saidbody including a generally tubular shaped wall; and said slurry outletmeans including a generally conically shaped wall converging inwardly inthe direction of slurry flow, a baffle which extends angularly upwardly,and inwardly from a wall of said conically shaped slurry outlet meansand defines with said wall a limited access chamber around said foamoutlet means, said baffle including a vertically upwardly extendingplate on the distal end thereof; control means in flow communicationwith at least one of said inlet means and said outlet means forproviding and maintaining a positive pressure in said body aboveatmospheric pressure; for controlling the flow of said slurry to lessthan fill said body, thereby generating a slurry surface; and forretaining the slurry between said inlet means and said outlet means forsufficient duration to permit gas near the bottom of said body to risethrough said slurry to the surface thereof; and said control meansdirecting gases escaping from the slurry to pass from said body withsaid foam through said foam outlet means.
 3. A flotation module asdefined in claim 2 in which said baffle extends upwardly at an angle ofless than about 40° from the horizontal.